Martensite stainless steel

ABSTRACT

The present invention relates to a martensite stainless steel with excellent toughness and used as oil well pipes. The martensite stainless steel according to the present invention comprises, by mass percent, 0.15 to 0.21% of C, 0.05 to 1.00% of Si, 0.3 to 1.00% of Mn, 10.5 to 14.00% of Cr, not greater than 0.010% of Al, not greater than 0.050% of P, not greater than 0.010% of S, 0 to 0.050% of Ti, 0 to 0.50% of V, 0 to 0.020% of Nb, not greater than 0.1% of N, and the balance being Fe and impurities, and the amount of Cr carbonitride precipitating at grain boundary is not greater than 5%.

TECHNICAL FIELD

The present invention relates to a martensite stainless steel with excellent toughness, and particularly, a martensite stainless steel for seamless steel pipes used as oil well pipes.

TECHNICAL BACKGROUND

Martensite steel such as API-13Cr, which is mainly used for oil well pipe, is required to have a yield strength at not less than 80 ksi (552 MPa). In order to meet this requirement, the C content is generally set round 0.2% in the composition design. Hereinafter “%” relating to the component content (exclusive of the amount of Cr carbonitride precipitating at grain boundary) represents “mass %”. The high contents of C, Cr and high strength degrade the toughness of products. Therefore, how to realize high-toughness 13Cr martensite stainless steel has become an important technical problem, and various methods have been proposed.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a martensite stainless steel comprising about 0.2% of C and having excellent toughness.

As to be described in details as follows, the present inventors have defined the contents of various components of the steel within respective reasonable ranges, and defined the amount of carbonitride precipitating at grains boundary, and thus realize the above-stated object.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a micrograph of a steel according to the invention; and

FIG. 2 is a micrograph of a steel falling outside the invention.

BEST MODE OF THE INVENTION

The present invention purposed that the toughness of the martensite stainless steel depends on the amount of carbonitride precipitating at grain boundary, and carried out the experiments, thus having confirmed the following the facts.

-   -   (1) The addition of Al significantly affects the toughness of         the steel pipe products.     -   (2) A great amount of Cr carbonitride precipitating at grain         boundary degrades the toughness.     -   (3) The precipitation of carbonitride at grain boundary can be         suppressed by controlling the addition of Al.

Conditions relating to the martensite stainless steel according to the present invention are described as follows.

1. The reasons for limiting the chemical components of the martensite stainless steel of the present invention are described as follows.

C: In the present manufacturing method, in order to obtain the appropriate strength, yield ratio and hardness, it is necessary to limit the range of the C content. If the C content is less than 0.15%, the strength of steel cannot meet the requirement. However, if the C content is in excess of 0.21%, it is difficult to regulate the yield ratio and the hardness. Therefore, the C content is defined within a range of 0.15 to 0.21%.

Si: Si is used as a deoxidizer of the steel. There are two sorts of deoxidizing methods, i.e., Si deoxidizing and Al deoxidizing. When using the Al deoxidizing method, a large amount of Al would remain in steel, and thus resulting in the decrease of toughness. Therefore, in order to control the content of Al, it is necessary to actively utilize Si-deoxidization. If the Si content is less than 0.05%, its effect cannot be realized, and if the Si content exceeds 1.00%, the toughness would be impaired. In consideration of the toughness, the Si content is preferably controlled at not greater than 0.75%, and more preferably 0.20 to 0.35%.

Mn: Mn is an element for improving the strength of the steel. Furthermore, similar to Si, Mn can act as a deoxidizer, and fix S contained in the steel in the form of MnS to ameliorate the hot workability. If the Mn content is less than 0.3%, these effects cannot be realized, and if the Mn content exceeds 1.00%, the toughness would be impaired.

Cr: Cr is a basic component for improving the corrosion resistance of the steel. In particularly, if the Cr content exceeds 10.5%, the corrosion resistance against pitting corrosion and crack corrosion can be ameliorated, and the corrosion resistant in CO₂ environment also can be improved significantly. On the other hand, since Cr is the element to form the ferrite, when its content exceeds 14.00%, it is easy to formδferrite in elevated temperature working process and degrade the hot workability. Further, the excessive addition of Cr would increase the manufacturing cost. Therefore, the Cr content is defined within a reasonable range of 10.5 to 14.00%, and more preferably 12.40 to 13.10%.

P: P is an impurity contained in the steel. The high content of P would cause the decrease of the toughness after thermal treatment. Therefore, the allowable upper limit of P is controlled at 0.050%, and preferably controlled as low as possibly.

S: S is an impurity that degrades the hot workability. Therefore, the S content is preferably controlled as low as possibly. The allowable upper limit is controlled at 0.010%, and more preferably at not greater than 0.005%.

N: N is a stabilizing element of austenite that meliorates the hot workability. Therefore, the N content is controlled at not greater than 0.1%, and preferably not greater than 0.035%.

Al: Al is the most important component of the present invention, and the control of its addition is really important. When the Al content exceeds 0.010%, the carbonitrides would precipitate excessively to impair the toughness. Therefore, the Al content is defined at not greater than 0.010%. Furthermore, in order to decrease the Al content, the Si deoxidizing method is preferably used in manufacturing method of steel since Al is an indispensably added element in the Al deoxidizing method.

V, Ti and Nb can form carbide and nitride by combining with C and N to improve the strength of the steel. Therefore, these elements are added in accordance with the requirements. Since the excessive addition of these elements would increase the cost, the V content is defined within a range of 0 to 0.50%, the Ti content is defined within a range of 0 to 0.050%, the Nb content is defined within a range of 0 to 0.020%. In addition, the content “0” in the present specification represents the situation that such an element is not added actively.

Furthermore, the other elements including Ni, Mo, Cu, Ca, B may also be contained in the steel.

Cr carbonitride amount: not greater than 5%

If the coarse carbonitrides of Cr precipitates at grain boundary, the bonding at grain boundary would be decreased and the impact resistance would be declined. Further, since the carbonitrides first precipitate at grain boundary, the precipitation amount of themselves must be decreased so as to improve the toughness. Therefore, the amount of carbonitrides precipitates at grain boundary is controlled at not greater than 5%.

The amount of Cr carbonitride precipitating at grain boundaries is defined according to the area ratio of the Cr carbonitride in a 5000-fold photo of the steel extraction replica.

EXAMPLES

Stainless steel pipes with the components shown in table 1 as the example of the present invention were prepared with the following steps. First, the billet was heated to the temperature of 1200° C. or higher, then pierced by piercer to obtain a pipe. Next, the pipe was heated to a predetermined temperature in a reheating furnace, and subjected to finish rolling with a mandrel mill and a reducer so as to obtain a predetermined size (outer diameter: 177.8 mm, wall thickness: 10.36 mm). After that, the pipe was air cooled to the room temperature at the speed of 2° C./second, and then subjected to the thermal treatment. The conditions of the thermal treatment were as follows: quenching was carried out for 960° C. 10 minutes; air cooling was carried out at the speed of not greater than 5° C./second; and tempering was carried out by regulating the tempering temperature after quenching so as to make the strength satisfy APIL80 grade (yield stress is 552˜656 MPa).

Test pieces of API 5CT standard were cut from the pipe manufactured by the method described above, and tensile strength, yield strength and average hardness of these bowtype test pieces were measured and charpy impact tests (form: 2 mm of V type with notch of 10 7.5 mm) were carried out thereon.

After preparing the extraction replicas, the carbonitrides precipitating at grain boundary were observed with a transmission electron microscope (TEM), and 5000-fold photos thereof were taken, and then the area ratio of the Cr carbonitride were calculated.

COMPARATIVE EXAMPLES

Stainless steel pipes with the components shown in table 1 as the comparative example of the present invention were prepared with the steps shown in the examples, and were subject to the same tests as shown in the example.

EFFECTS OF THE INVENTION

As shown by the test results in table 1, the 13Cr martensite seamless steel of the present invention has excellent toughness, and can be used in the seamless steel pipe to meet the increasing requirements of the corrosion resistance oil wall pipe and so on nowadays. TABLE 1 Chemical Component (mass %) No. C Sl Mn P S Cr Ti V Nb Al N  1 0.19 0.25 0.91 0.014 0.0036 12.50 0.004 0.04 0.002 0.001 0.0482  2 0.20 0.21 0.48 0.013 0.0007 12.12 0.002 0.17 0.001 0.001 0.0685  3 0.17 0.23 0.91 0.013 0.0024 12.57 0.010 0.04 0.007 0.006 0.0274  4 0.18 0.35 0.89 0.012 0.0006 13.07 0.012 0.04 0.001 0.008 0.0761  5 0.20 0.23 0.64 0.018 0.0005 13.55 0.007 0.20 0.016 0.002 0.0474  6 0.19 0.18 0.82 0.014 0.0046 12.49 0.006 0.04 0.001 0.001 0.0386  7 0.19 0.27 0.90 0.013 0.0012 12.57 0.004 0.03 0.002 0.026* 0.0494  8 0.20 0.22 0.52 0.014 0.0005 12.49 0.003 0.05 0.001 0.012* 0.0521  9 0.20 0.21 0.78 0.014 0.0006 12.59 0.007 0.13 0.001 0.016* 0.0361 10 0.20 0.25 0.91 0.016 0.0011 12.56 0.010 0.04 0.001 0.023* 0.0457 11 0.20 0.24 0.84 0.014 0.0005 12.54 0.022 0.18 0.001 0.025* 0.0478 12 0.20 0.26 0.92 0.015 0.0009 12.51 0.013 0.04 0.001 0.028* 0.0611 13 0.22* 0.33 1.23* 0.016 0.0015 12.80 0.003 0.08 0.003 0.001 0.0250 14 0.19 1.13 0.70 0.015 0.0043 12.50 0.024 0.12 0.002 0.002 0.0277 15 0.19 0.37 0.67 0.013 0.0058* 12.60 0.030 0.11 0.004 0.007 0.0495 16 0.19 0.33 0.71 0.015 0.0024 12.50 0.008 0.10 0.002 0.006 0.1013* 17 0.19 0.87 0.59 0.025* 0.0031 12.60 0.003 0.07 0.001 0.005 0.0236 18 0.20 1.20* 0.73 0.018 0.0008 13.00 0.009 0.13 0.000 0.001 0.0765 19 0.20 0.34 0.72 0.016 0.0079* 12.70 0.004 0.12 0.003 0.002 0.0263 20 0.18 0.26 0.49 0.016 0.0021 12.73 0.026 0.08 0.002 0.008 0.0514 21 0.19 0.23 0.88 0.014 0.0084* 12.50 0.015 0.05 0.002 0.004 0.0451 22 0.20 1.03* 1.04* 0.020 0.0005 12.50 0.012 0.02 0.025 0.002 0.0253 23 0.14* 0.34 0.08* 0.010 0.0034 13.10 0.007 0.06 0.001 0.001 0.0325 Average Area Temper intension Charpy ratio of Cr temp YS TS EL rigidity vTrs carbonitride No. [° C.] [MPa] [MPa] [%] [HRC] [° C.] [%]  1 715 598 769 29.2 21.2 −18 2 Present  2 735 572 764 30.2 20.9 −10 2 invention  3 720 577 772 28.4 20.7 −6 4  4 705 595 781 28.0 21.6 −8 5  5 750 581 770 30.1 21.8 −16 2  6 705 591 775 28.0 21.6 −22 3  7 715 597 768 30.3 20.7 18 12 Comparative  8 710 585 782 29.6 21.6 14 7 example  9 725 578 774 28.6 20.7 19 8 10 705 605 797 27.6 22.3 26 10 11 735 592 791 29.2 21.9 17 10 12 705 598 800 28.0 22.6 24 11 13 725 586 764 29.2 21.5 27 2 14 725 602 770 28.4 21.5 34 2 15 720 564 758 30.6 22.2 29 4 16 725 559 748 30.2 20.4 16 5 17 735 561 755 29.4 21.3 29 4 18 725 555 759 29.2 22.2 31 2 19 735 561 764 28.9 22.3 36 3 20 730 562 759 29.0 20.5 18 5 21 710 560 751 28.4 21.1 34 4 22 740 568 763 28.8 21.3 24 2 23 700 559 751 28.4 21.2 8 1 *YS represents the Yield Strength; TS represents the Tensile Strength; EL represents the Elongation. The chemical components further include balance composed of Fe and impurities. 

1. Martensite stainless steel comprising, by mass percent, 0.15 to 0.21% of C, 0.05 to 1.00% of Si, 0.3 to 1.00% of Mn, 10.5 to 14.00% of Cr, not greater than 0.010% of Al, not greater than 0.050% of P, not greater than 0.010% of S, 0 to 0.050% of Ti, 0 to 0.50% of V, 0 to 0.020% of Nb, not greater than 0.1% of N, and the balance being Fe and impurities, wherein the amount of Cr carbonitride precipitating at grain boundary is not greater than 5%. 